1. Field of the Invention
This invention relates in general to gaseous measurement devices and in particular to a new and useful electrochemical measurement cell for measuring gases.
The invention concerns an electrochemical measurement cell with an anode and a cathode, which are placed in an acid electrolyte for galvanic measurement of oxidizable gases, especially oxygen, and accommodated in a housing, which closes off the electrolyte volume against the gas atmosphere to be investigated by means of a permeable membrane.
Such measurement cells are used preferably to detect oxygen in air, whereupon the oxygen diffusing through the membrane into the electrolyte is reduced. At the cathode, oxygen is reduced to water, protons being consumed and electrons given off. The protons necessary for this must be provided by the electrolyte.
As acid electrolytes, one employs monocarboxylic acids such as acetic acid, propionic acid, or butyric acid in aqueous solution. The degree of dissociation of these acids is so slight, however, that the conductivity needed for an oxygen measurement cannot be achieved unless salts of these acids are added.
A similar electrochemical measurement cell with an acid electrolyte is described in U.S. Pat. No. 4,495,051.
In the known electrochemical measurement cell, protons are continuously consumed during the measurement, which must be taken from the electrolyte. This proton capacity is quickly depleted in the case of the monocarboxylic acids which are used. When the proton reserve is exhausted, no further operation of the measurement cell is possible, so that its lifetime is very restricted. The continuing proton consumption produces a shifting of the pH value of the electrolyte solution in the alkaline direction. If this value exceeds pH 7, poorly soluble bicarbonates and ultimately carbonates may be formed, which impair the functioning of the measurement cell. A passivation layer is then formed on the anode and a carbonate layer between the cathode and the diffusion membrane, resulting in a larger overvoltage and, thus, lower conductivity.
Addition of salts of the particular acids, such as alkaline metal and/or ammonium salts, although increasing the conductivity by virtue of their high degree of dissociation, does not produce a larger proton capacity of the electrolyte. Furthermore, there is a limit to the quantity of salts that can be added, since these increase the pH of the solution by virtue of their basic nature, which has an undesirable effect on the CO.sub.2 stability of the sensor, especially if the pH limit of 7 is exceeded. In many applications, especially during measurement of oxygen in respiration gas, a large proportion of CO.sub.2 in the investigated gas is to be expected. Since the membrane closing off the electrolyte from the surrounding gas atmosphere may be permeable to CO.sub.2, the contact between the electrolyte and the CO.sub.2 present in the air of the environment results in further formation of carbones and bicarbonates. This is manifested by a shifting of the pH to lower values, resulting in a deposition of lead oxide and lead carbonate on the anode surface in the case of lead anodes. Since the poor solubility of lead carbonates may result in precipitation of salts, the function of the cathode is impaired, as additional diffusion barriers are presented to the diffusing oxygen. Furthermore, the formation of crystals exerts a mechanical pressure on the diffusion membrane, producing a stress on the thin diffusion membrane.
Because of the large vapor pressure of the acid electrolytes employed thus far, a not insignificant diffusion of the electrolyte occurs through the membrane, and also through the walls of the housing, inasmuch as the walls consist almost always of plastic in recent time.
All these undesirable features are aggravated even further when a miniaturization of the outside dimensions of the measurement cell is desired. In such case, there is an unfavorable shifting in the ratio of the surface of the electrodes and inner housing to the volume of the electrolyte, whereupon the surface-related perturbations, such as diffusion from the membrane, passivation of the anode surface, increase in the electrolyte resistance, are intensified.